In general, in the process of hot pressing or thermal compression bonding in manufacturing the laminated board, as shown in FIG. 10, the following method is used, that is, a mirror-plate 12 is put on upper and lower surfaces of a laminated board 11 which is an object, and a release sheet 13 and a cushion material 14 are interposed on upper and lower surfaces of the mirror-plate 12 so as to be sandwiched by hot platens 15, and pressure and heat are applied thereto. In order to provide the laminated board with high precision, it is necessary to uniformly apply the pressure and heat over an entire surface of the laminated board material 11.
Thus, the cushion material 14 carries out a function of reducing unevenness of heat and pressure of the hot platen 15 so that the pressure and heat can be applied uniformly over the entire surface. As the cushion material 14, various kinds of materials such as kraft paper, organic fibers or inorganic fibers combined by a binder, a rubber, non-woven fabric, a laminated material of rubber and non-woven fabric and the like are used. When the cushion material 14 is made of a fibrous material, bits of fibers are separated from the cushion material 14 in some cases. When the bits of fibers are attached on the laminated board material 11 or the mirror-plate 12, which causes dents or scars on the laminated board material 11.
When the cushion material 14 is made of a material having a low release property, the cushion material 14 sticks to the mirror-plate 12 in some cases. Thus, the release sheet 13 carries out a function of preventing the bits of the fibers from being separated from the cushion material 14 and floating, and a function of preventing the cushion material 14 from sticking to the mirror-plate 12. Meanwhile, when the surface of the cushion material 14 itself has a release property, the release sheet may not be used. According to the present invention, a term of the hot-press sheet means it includes both release sheet 13 and cushion material 14.
As a conventional hot-press release sheet, a sheet formed of a single synthetic resin film or a sheet formed of a composite material comprising a sheet-shaped base material and a release coating film formed on the base material are known. Japanese Unexamined Patent Publication No. 9-182998 (patent document 1) is a well-known document about a release sheet comprising a composite material. According to the patent document 1, the release sheet comprises a base material layer and a release layer formed on a surface of the base material layer, and the release layer is formed of a heat-resistant release coating or a heat-resistant release film.
Japanese Unexamined Patent Publication No. 6-322326 (patent document 2) is a well-known document about improvement of a release surface material of a cushion material. The patent document 2 discloses a cushion material in which a heat-sensitive adhesive sheet is provided by applying an unvulcanized fluoro rubber to one surface (back surface) of a sheet-shaped base material and by applying heat-resistant coating to the other surface (front surface) thereof and laminated in such a manner that the unvulcanized fluoro rubber surface may be in contact with an object, and they are bonded so as to be integrated.
Each of the release sheet described in the patent document 1 and the cushion material described in the patent document 2 comprises the sheet-shaped base material and the release coating film comprising the heat-resistant coating formed on the base material.
According to the conventional hot-press sheet, when the heat-resistant coating was applied onto the surface of the base material and dried, thin parts called sinks or dents were likely to be generated in the coating film. These sinks or dents mean a phenomenon in which a few thin parts having a diameter about 1 mm are generated in the hot-press sheet per 1 m2 and it was difficult to eliminate these sinks or dents. The hot-press sheet needs to have durability in which it can be used in pressing processes several hundred of times. If the hot-press sheet has the sinks or dents, while it is repeatedly used, a crack caused by the sink or dent is generated or the base material at the sink or the dent is exposed because of abrasion of the surface, which lowers the durability. In addition, when the hot-press sheet having the sinks or dents on the surface is bent at the time of handling such as transporting, a crack is likely to be generated because of the sink or dent.
The inventors of the present invention confirmed in the experiments that when a powder filler was mixed in the heat-resistant coating, the sink or dent could be effectively prevented. However, it was found also that when the powder filler was mixed in a normal way, general requirement characteristics for the hot-press sheet such as abrasion resistance, release property, flexibility, coating workability of the coating film were likely to deteriorate, so that it is a new issue to prevent the sink or dent without lowering the above requirement characteristics.